The cell surface antigen CD38 is a multifunctional ectoenzyme that acts as an NAD(+) glycohydrolase, an ADP-ribosyl cyclase, and also a cyclic ADP-ribose hydrolase. The extracellular catalytic domain of CD38 was expressed as a fusion protein with maltose-binding protein, and was crystallized in the complex with a ganglioside, G(T1b), one of the possible physiological inhibitors of this ectoenzyme. Two different crystal forms were obtained using the hanging-drop vapor diffusion method with PEG 10,000 as the precipitant. One form diffracted up to 2.4 A resolution with synchrotron radiation at 100 K, but suffered serious X-ray damage. It belongs to the space group P2(1)2(1)2(1) with unit-cell parameters of a = 47.9, b = 94.9, c = 125.2 A. The other form is a thin plate, but the data sets were successfully collected up to 2.4 A resolution by use of synchrotron radiation at 100 K. The crystals belong to the space group P2(1) with unit-cell parameters of a = 57.4, b = 51.2, c = 101.1 A, and beta = 97.9 degrees, and contain one molecule per asymmetric unit with a VM value of 2.05 A(3)/Da.